A CCD is often utilized as a line sensor for this kind of the inspection system. The line sensor has an arrangement wherein multiple photodiodes are arranged in line and CCDs are arranged in parallel to correspond to each of the photo diodes. Then, electric charge generated due to exposure of the photodiode is charged to the CCD and the charged electric charge is read out respectively so that a line of the image data is produced.
Since it is not possible for this kind of line sensor to conduct a next exposure until an electric charge charged to the CCD is read out and reset, it is relatively difficult to significantly shorten an interval of exposure, namely, an imaging interval, if a reset time is also included. However, by conversely utilizing the property that an electric charge is stored in succession until the CCD is reset, recently a line sensor has been developed that employs a TDI (Time Delay Integration) method which can conduct sensing at high speed and high accuracy.
The TDI method is to synchronize a moving speed of an object projected to the CCD in a direction of a column with a timing of transferring the electric charge in a vertical direction and to conduct an integral exposure by a number of vertical steps (for example, “M” steps) by utilizing a property that the electric charge is vertically transferred by a unit of a column at a time of reading out the electric charge, where the CCD is arranged in two-dimensions, so that a sensitivity by “M” times of an ordinary linear image sensor can be realized. In short, the identical position of the object is imaged at “M” times and all of the imaged images are overlapped so as to improve the sensitivity.
Patent document 1: Japan Patent Laid-open number 2004-251878